Non-detection of ⁶Li in Spite plateau stars with ESPRESSO

The detection of 6Li in Spite plateau stars contradicts the standard big bang nucleosynthesis prediction, known as the second cosmological lithium problem. We measure the isotopic ratio 6Li/7Li in three Spite plateau stars:HD 84937, HD 140283, and LP 815-43. We use 3D non-local theodynamic equilibrium radiative transfer and for the first time apply this to high-resolution, high signal-to-noise ratio data from the ultra-stable ESPRESSO/Very Large Telescope spectrograph. These are among the best spectra ever taken of any metal-poor stars. As the measurement of 6Li/7Li is degenerate with other physical stellar parameters, we employ Markov chain Monte Carlo methods to find the probability distributions of measured parameters. As a test of systematics, we also use three different fitting methods. We do not detect 6Li in any of the three stars, and find consistent results between our different methods. We estimate 2upper limits to 6Li/7Li of 0.7, 0.6, and 1.7 per cent, respectively, for HD 84937, HD 140283, and LP 815-43. Our results indicate that there is no second cosmological lithium problem, as there is no evidence of 6Li in Spite plateau stars.